This invention provides an advanced medical oxygen concentrator process and system. More specifically, the invention achieves a compact, light weight and low power medical oxygen concentrator using a fast PSA (pressure swing adsorption) cycle and advanced adsorbents. The invention provides significant system improvements and cost savings over commercial stationary medical concentrators. Also, when integrated with a conserver, the invention provides a truly portable unit.
A growing number of people need oxygen to alleviate respiratory insufficiency. Although home oxygen can be provided by liquid or high pressure cylinders, more recently medical oxygen concentrators have become a cost effective and preferred system.
Most oxygen concentrators are based on pressure swing adsorption (PSA)including vacuum swing adsorption (VSA) or vacuum pressure swing adsorption (VPSA). PSA is a well-known art for separating oxygen from air for various applications ranging from a few liters per minute (LPM) in medical concentrators to several hundred tons per day of oxygen (TPDO) in industrial scale plants.
While a medical concentrator and large scale industrial plant share the same PSA fundamentals, it is the nature and scale of their applications that differentiate the two. Components of a PSA system contribute differently between the medical concentrator and the large scale plant to the overall cost. For example, while adsorbent and vessel normally represent a large contribution to the overall cost in the large scale plant, the compressor is the single largest component for medical concentrators. In addition, because a medical concentrator is for home use, its size and weight are critical performance parameters, especially for a portable unit. Conversely, in a large scale plant, system size and weight are not particularly important other than their effects on the overall oxygen cost.
It is well known that the overall PSA performance depends primarily on the inter-relationship of three parameters: BSF (the amount of adsorbent required to produce a given quantity of oxygen per unit of time (lbs/TPDO)), recovery (where oxygen recovery is defined as the ratio of the oxygen in the product to the oxygen in the feed) and power consumption (the overall power consumed in a process per unit of product produced, also known as specific power). Thus, while a small BSF may reduce the adsorbent cost, this may be offset by decreased product recovery and higher power consumption. Power consumption becomes a significant issue, especially with portable units powered by a battery.
Medical concentrator design must consider not only the cost and power consumption, but also the system size, weight and comfort. Most efforts in the prior art of oxygen concentrators have been focused on developing small and economic systems.
Typical commercial concentrators use zeolite adsorbents such as 5A, 13X or Oxysiv-7 (LiX (SiO2/Al2O3=2.5) available from UOP, Des Plaines, Ill. USA). Highly exchanged LiX adsorbents are illustrated by Chao in U.S. Pat. No. 4,859,217.
A more advanced LiX adsorbent with SiO2/Al2O3=2.0 was recently disclosed by Chao and Pontonio (WO 99/43415). This adsorbent was exploited by Ackley and Leavitt (WO 99/43416), and Ackley and Smolarek (WO 99/43418) to achieve fast PSA cycles in the production of oxygen.
Norman R. McCombs(U.S. Pat. No. 5,474,595) disclosed a medical concentrator based on a two-bed PSA and having a capacity control system disposed upstream of the compressor for power reduction.
Charles C. Hill and Theodore B. Hill (U.S. Pat. No. 5,366,541) disclosed a medical concentrator employing a rotary distribution valve instead of a number of conventional solenoid valves. This design simplified the conventional concentrator system and was more compact and reliable.
R. H. Kaplan et al. (AIChE Meeting, Nov. 7, 1989, San Francisco) traced developments in the design of concentrators and selected a rapid PSA (RPSA) system. Using a three-bed system and small adsorbent particles (40xc3x9780 beads, or diameter xcx9c0.25 mm), the cycle time was reduced to as low as 2.4 s. The present inventors calculate that bed size factor (BSF), was about 200 lbs/TPDO when operating at an adsorption pressure of 30 psig and a desorption pressure that is atmospheric, (e.g. an adsorption/desorption pressure ratio of xcx9c3). The corresponding adsorbent weight based on a standard 5 LPM capacity was then determined to be about 2.2 lb. This BSF (e.g. 2.2 lbs/5 LPM oxygen)was about two times smaller than that obtained by other conventional PSA concentrators. However, the achieved oxygen recovery was only about 25%. This recovery is very low compared with large scale VPSA processes which achieve as much as xcx9c60% to 70%. Because of the low recovery a large air compressor is required. Also, the pressure drop in this RPSA system was large, about 8 psi/ft at 1 fps superficial velocity compared with less than 1 psi/ft in large O2 PSA plants. The low recovery and high pressure drop result in a concentrator having a relatively high power consumption.
Stanley Kulish and Robert P. Swank (U.S. Pat. No. 5,827,358) disclosed another rapid PSA oxygen concentrator. It employed at least three adsorbent beds, and a process cycle of approximately 1xcx9c2 seconds (s) for the adsorption step and 5xcx9c10s for the desorption step. Using a six-bed configuration, the rapid cycle allowed for a BSF we estimate to be about 125 lb/TPDO at a pressure ratio of about three. Thus the adsorbent inventory is about 1.3 lb for 5 LPM capacity system. No recovery result was disclosed.
Sircar, in U.S. Pat. No. 5,071,449 disclosed a single bed RPSA cycle having a continuous feed and a cycle time ranging from six to sixty seconds.
Typical parameters for prior art oxygen concentrators are summarized in Table 1.
The power set forth in the Table is larger than that required for large scale industrial PSA systems, where the typical power is 10kw/TPD or less. The power is lower for at least one of the following reasons:
1. higher separation power of superatmospheric PSA;
2. lower efficiency of smaller scale blowers; and
3. higher compression power of faster cycle processes.
While so-called xe2x80x9cportablexe2x80x9d concentrators having a capacity of about 2-3 lpm of oxygen do exist, such machines typically weigh more than about 20 lbs. excluding battery. Thus, there is a need to reduce the size and weight of such portable medical oxygen concentrators.
An additional problem associated with medical oxygen concentrators is that a large part of product oxygen is wasted if oxygen flow is continuously provided to the patient, since oxygen is only brought to the patient""s lungs during inhalation (about ⅓ time of the breathing cycle).
Chua et al. U.S. Pat. No. 5,735,268 disclose the use of a conserver to save breathing oxygen from a source such as liquid oxygen tank to the respiratory patient.
Sato et al. in U.S. Pat. No. 4,681,099 teaches the combination of a concentrator and a conserver where an oxygen buffer tank connecting the concentrator and the conserver makes the initial oxygen flow higher than the steady flow of each inhalation phase.
The present invention combines a very fast pressure swing adsorption oxygen cycle with a high-rate adsorbent to achieve significant improvements over commercial/prior art medical oxygen concentrators. In most preferred embodiments, the cycle time may be as short as xcx9c4 s, and the adsorbent inventory and vessel volume are decreased by a factor of at least about seven when compared with current commercial medical concentrators. In more preferred embodiments, the oxygen recovery achieved is greater than 50%. The result is a concentrator having a size, weight and power consumption that are significantly reduced when compared to the current state of the art.
In a further preferred embodiment of the present invention, the inventive portable concentrator system and process of the invention is further integrated with a conserver.
Product purity for the medical concentrators of the invention ranges from about 85 to 95% oxygen.